1. Field of the Invention
The present invention relates to a method of producing hydroxypivalaldehyde and/or a dimer thereof by subjecting isobutyl aldehyde to reaction with formaldehyde. More specifically, the present invention relates to a method of efficiently and simply producing high-purity hydroxypivalaldehyde and/or the dimer thereof by separating the hydroxypivalaldehyde and/or the dimer thereof from a reaction solution by a crystallization process.
2. Description of the Prior Art
In general, hydroxypivalaldehyde (3-hydroxy-2,2-dimethyl propanal, hereinafter referred to as HPA) is synthesized by an aldol condensation reaction of isobutyl aldehyde with formaldehyde in the presence of a basic catalyst. The aldol condensation reaction can proceed either in acidic conditions or basic conditions. However, HPA has a carbonyl group and a hydroxyl group in a single molecule thereof, so the condensation of dimers into tetramers occurs under acidic conditions. Thus, the aldol condensation reaction is generally carried out under basic conditions as mentioned above (Japanese Patent Application Laid-Open No. 07-215904 and Japanese Patent Application Laid-Open No. 2000-26356).
After the reaction is completed, low boiling point components, such as unreacted isobutyl aldehyde and formaldehyde, are distilled off to thereby obtain an HPA-containing aqueous solution. HPA has been often used as a synthetic intermediate of an organic compound such as neopentylglycol or spiroglycol, and such the liquid resulting from the reaction is often used in subsequent steps without being purified (Japanese Patent Application Laid-Open No-01-299239 and Japanese Patent Application Laid-Open No. 2005-29563).
As for HPA, there is an equilibrium relationship between a monomer and a dimer as represented by the chemical formula (I) (Journal of the Chemical Society, Perkin Transactions II, vol. 3, page 189-192, 1978). HPA obtained as a crystal by crystallization and purification is a dimer. Japanese Patent Application Laid-Open No. 01-299239 and the like disclose that the dimer is as reactive as the monomer.

In addition, methods of obtaining high-purity HPA are disclosed in Japanese Patent Application Laid-Open No. 51-68514 and Japanese Patent Publication No. 06-29206, in which the HPA-containing aqueous solution is diluted with the addition of water and then purified by crystallization, followed by solid-liquid separation to thereby obtain the high-purity HPA.
In other words, Japanese Patent Application Laid-Open No. 51-68514 discloses that the crystallization process is carried out such that water is added to a reaction solution for aldol condensation to keep the total content of the HPA and/or the dimer thereof within the range of 23 to 30% by mass after distilling isobutyl aldehyde off from the reaction solution. However, when crystallization was carried out at such high concentrations after the reaction solution was cooled down to 15 to 20° C., it produces a slurry product with an extremely high viscosity or one without fluidity. Therefore, there arises a disadvantage that it is very difficult to be handled industrially. Further, Japanese Patent Application Laid-Open No. 51-68514 discloses that a large amount of the HPA and/or the dimer thereof remain/remains in a liquid obtained by the solid-liquid separation of the slurry, so the HPA and/or the dimer thereof can be recovered by extracting with isobutyl aldehyde and circulating an organic phase in the step of low-boiling distillation, while isobutyl aldehyde dissolved in an aqueous phase is also recovered by distillation. However, because of a large amount of isobutyl aldehyde distilled in a distillation column, it is energetically disadvantageous.
Further, Japanese Patent Publication No. 06-29206 discloses that part of filtrate and wash solutions obtained in the steps of solid-liquid separation and washing is circulated into an aldol condensation reactor and the distillation column, while the remaining part is subjected to a deamination processing to utilize it as addition water and rinse water in the crystallization process. In this case, however, formic acid, which is a by-product from the aldol condensation, is accumulated by circulation, causing a decrease in a pH level of the solution to be subjected to the crystallization. Therefore, there is a disadvantage in that the HPA and/or the dimer thereof are/is not easily crystallized. Further, even in the method of Japanese Patent Publication No. 06-29206, because a large amount of the liquid is circulated in the step of distillation, there are disadvantages in that the distillation step is highly loaded, it is energetically disadvantageous, and the steps are complicated since regeneration of the ion exchange resin used in a deamination processing is necessary.